1. Field of the Invention
The present invention relates to a development device incorporated in an image forming apparatus such as a copying machine, a facsimile machine or a printer and to an image forming apparatus including such a development device. More particularly, the invention relates to a method for controlling the driving of a development device that employs a touchdown development method in which a two-component developer composed of a magnetic carrier and toner is used, and charged toner on a development roller is only retained and then an electrostatic latent image on an image carrying member is developed in a contact or noncontact manner.
2. Description of the Related Art
Conventionally, the following two methods are known as dry toner development methods for use in an image forming apparatus employing an electrophotographic process: one is a one-component development method in which no carriers are used; and the other is a two-component development method in which a two-component developer whereby non-magnetic toner is electrically charged with a magnetic carrier is used, and an electrostatic latent image on an electrostatic-latent-image carrying member (a photoconductor) is developed with a magnetic brush composed of the toner and the carrier formed on a development roller.
Since an electrostatic latent image on an electrostatic-latent-image carrying member is not disturbed by a magnetic brush, the one-component development method is suitable in seeking higher image quality. In the one-component development method, however, toner is electrically charged with a charge roller, and the thickness of a layer on a development roller is restricted by an elasticity-restriction blade, with the result that an additive for the toner adheres to the charge roller and this reduces charging capacity. Thus, it is difficult to stably maintain the amount of charge on toner. Moreover, the toner may adhere to the restriction blade to form an uneven toner layer on the development roller, and this results in degraded image quality.
In the case of color printing where color images are superimposed, since toner is required to be transparent, it is necessary to use non-magnetic toner. Thus, full-color image forming apparatuses frequently employ the two-component development method in which toner containing no carrier components is only charged and transferred. In the two-component development method, the amount of charge on toner can be stably maintained for a long period, and this is suitable in obtaining a longer life of toner. However, the two-component development method is disadvantageous in terms of image quality due to adverse effects of the magnetic brush.
One proposed solution to these problems is to employ a hybrid (touchdown) development method in which when developer is transferred with a magnetic roller onto a development roller that is arranged so as not to make contact with an electrostatic-latent-image carrying member (a photoconductor), non-magnetic toner is only transferred, while magnetic carrier is being left on the magnetic roller, onto the development roller to form a thin toner layer, and then the toner is adhered to a latent image on the electrostatic-latent-image carrying member (the photoconductor) by an alternating-current electric field.
With this technology, since the two-component development method described above is adopted in consideration of a longer life of toner in the area where the toner is electrically charged and the one-component development method in which the toner is only adhered to a photoconductor in a contact or noncontact manner in order to achieve higher image quality is thereafter adopted in the area where development is performed, it is possible to take advantage of both the one-component development method and the two-component development method. Thus, the hybrid (touchdown) development method is the most suitable for full-color image forming apparatuses required to have higher image quality and a longer life.
Disadvantageously, however, even with the touchdown development method, since the area exists in which the two-component development method where the magnetic carrier is used is adopted, when the resistance of the developer decreases under high-humidity conditions, the carrier deteriorates due to prolonged driving of the development device or other problems arise, the carrier is transferred through the development roller to the electrostatic-latent-image carrying member, with the result that white or black spots appear in a printed image; this so-called carrier development is more likely to occur. The carrier development is more likely to occur particularly in a solid image or a half-tone portion of an image; this results in an extremely serious problem from an appearance standpoint and a tactile standpoint.
To overcome these problems, various methods are proposed that prevent the occurrence of the carrier development. For example, patent document 1 (JP-A-2001-134066) discloses an image forming apparatus employing a two-component development method in which the carrier development is prevented by varying at least one of three factors, namely, the rotation speed of a developer carrying member (a development roller), the magnitude of the direct-current component of a development bias and the surface potential of a photoconductor.
According to the method disclosed in patent document 1, however, since it is necessary to decrease the magnitude of the direct-current component of the development bias, the surface potential of the photoconductor or to increase the rotation speed of the development roller under conditions in which the carrier development is more likely to occur, the occurrence of the carrier development can be prevented but the development performance may be degraded. Thus, it is likely that stable images cannot be obtained. In the method disclosed in patent document 1, the movement of the carrier from the development roller to the photoconductor is prevented in the two-component development method. Hence, it is not possible to apply the method disclosed in patent document 1 as it is to the touchdown development method, where the movement of the carrier from the magnetic roller to the development roller needs to be prevented.